There is an increasing demand for formed plastic pieces embodying a particular design which can be used in signs and similar articles, and, quite often, such pieces must be formed in only very limited quantities rather than mass produced. Equipment to form such plastic pieces must be relatively inexpensive to produce and easy to operate by untrained personnel, and it must be versatile enough to permit a large variety of totally different signs to be formed without requiring complicated and time consuming operating procedures.
Equipment is presently available for this purpose, a typical example of which is disclosed in Shuman U.s. Pat. No. 3,553,784, issued Jan. 12, 1971. Generally, this equipment includes an elevated heating oven, and a forming table which supports a vacuum forming mold and a frame arrangement for holding a sheet of plastic material which is melted by the elevated oven until plasticized, whereupon it is drawn by vacuum across the mold so as to conform to the contours thereof.
This equipment has been found to be satisfactory in operation, but it has several practical drawbacks. As noted above, such equipment must be capable of utilizing a variety of molds having different sizes and shapes, and while the equipment disclosed in U.S. Pat. No. 3,553,784, has this capability, it requires a rather significant inventory of relatively expensive mold supporting vacuum boxes, each designed for molds of different sizes, and considerable time is required to change from one mold supporting unit another.
More specifically, since the edges of the plastic sheet to be formed must be pulled down over all edges of the forming mold to assure that all surface contours of the mold are faithfully reproduced in the formed plastic, presently available equipment includes a vacuum box which rests on a reinforced platen, such vacuum box having peripheral dimensions that are similar to the corresponding dimensions of the mold and having vertical side walls which permit the mold to be supported atop the vacuum box while permitting the plastic holding frame to be lowered along the vertical sides of the vacuum box and thereby pull the edges of the plastic sheet to a position beneath the lowermost edges of the elevated mold. While this arrangement operates satisfactorily as indicated above, it will be appreciated that a separate vacuum box is required for each different mold size, and these vacuum boxes, in addition to being expensive to produce because they must be strong enough to withstand the vacuum imposed on the mold, are cumbersome to handle and require considerable storage space because of their dimensions. Moreover, the vacuum source must have a significant capacity because the interior of the forming mold itself, and finally, some difficulty is encountered in locating and fixing the vacuum box in place on the platen so that it will be properly oriented when the plastic holding frame is lowered thereabout. For example, it is common practice to use masking tape to hold the vacuum box in place, but masking tape is both expensive and time consuming to use.
Additionally, since the plastic holding frame must be capable of use with a variety of different mold sizes, it must have sides which can be adjusted to vary the size of the frame so that it will conform generally with the peripheral dimensions of the different molds. Heretofore, the four frame sides have been bolted together at selected points along the lengths thereof, and this arrangement required a considerable amount of time consuming labor to unbolt and rebolt all four sides each time the dimensions of the frame were altered to fit a particular mold.
In accordance with the present invention, a unique forming table and adjustable plastic holding frame are provided which eliminate the aforementioned drawbacks of existing plastic forming equipment.